1. Field of the Invention
The present invention relates to a controlling apparatus for an optical amplifier and a controlling method therefor, to be used in an optical communication system, such as a WDM (wavelength division multiplexing) transmission system.
2. Description of the Related Art
In the optical communication system, as is well known, long distance transmission has been realized by providing one or more of optical amplifiers located in each predetermined interval. For example, on a transmission line crossing the Pacific Ocean, tens to hundreds optical amplifiers have been disposed. Some optical amplifiers use an optical fiber, in which rare earth ions, such as Er (erbium) ions, Pr (praseodymium) ions, and Tm (thulium) are doped, as an amplification medium, wherein among them, as compared with others, there has been used frequently an EDF (erbium doped fiber) optical fiber, in which the erbium ions are doped, whereby high output power, a high gain, and a broad band can be obtained.
On the other hand, the amount of information, to be transmitted via networks, has been increasing rapidly, according to the popularization of the Internet, and a larger capacity of the transmission system has been required. And as one of the technologies to realize the larger capacity of the transmission system, the WDM transmission system has been already put into practice.
At this WDM transmission system, there is multiplexed a plurality of signal light (channels) whose wavelengths are different from one another, whereupon the multiplexed signal light is transmitted via one optical transmission line (optical fiber). Therefore, it is required for the optical amplifier being used in the WDM transmission system to be able to amplify a plurality of signal light all together.
And recently, in order to realize a more flexible network structure, an OADM (optical add-drop multiplex) apparatus that can add or drop each signal light (channel) individually has been put into practice, and the signal light power to be transmitted via the optical transmission line does not become constant. As mentioned above, when the signal light power has fluctuated due to the change in the number of channels, (this change naturally occurs also in case of some trouble having occurred at a part of the channels), signal light of other channels cannot be received and demodulated correctly.
Therefore, at a conventional optical communication system, even in the case where the number of signal wavelengths of the WDM signal has changed, there is carried out control such that the output power of each signal wavelength is kept constant, by controlling the amplification gain at the optical amplifier to be constant. This controlling method is referred to as AGC (automatic gain control).
Concretely, the output light power of the optical amplifier is monitored, and the amplification gain (actually, for example, pumping light power) of the optical amplifier is controlled to be constant, based on the monitored value.
However, since at the above-mentioned conventional controlling method (AGC), time of approximately several 10 ms is required from the time of change in the input light power up to the achievement of a suitable control, with the result that this required time places a limit on increase and decrease in the signal light wavelengths. And in the event of some disconnection attributed to an accident having occurred, there would be exerted adverse influences on signal levels of other wavelengths for a long period of time.
Therefore, at the AGC of the optical amplifier, it is desirable that the output light power is controlled to be constant in a short time by high speed-response to the change of the input light power in high speed. However, if the response speed of the AGC should be made high to a great extent without any limit, there is fear of occurrence of an oscillation phenomenon.
Therefore, as the high speed method at the AGC, conventionally, for example, there are technologies proposed by Japanese Patent Laid-Open (Kokai) HEI 9-200145 (hereinafter, referred to as Patent Document 1) and Japanese Patent Laid-Open (Kokai) HEI 7-221737 (hereinafter, referred to as Patent Document 2).
That is, first, the technology described in the Patent Document 1 has a structure in which signal light to be amplified is inputted to an optical amplifier (EDF) via an optical fiber having a predetermined delay time, and the supply of pumping light is started in a period between the time when the reach of the signal light at the input terminal of the optical fiber has been detected and the time when the signal light reaches the EDF (for example, refer to paragraphs [0034] to [0039] of the Patent Document 1). Thereby, it becomes possible that time requiring for pumping light control has a margin of above-mentioned delay time, and as a result, it becomes possible that the occurrence of optical surge is restrained and also the response speed of the AGC is enhanced.
On the other hand, at the technology described in the Patent Document 2, adjusting light for canceling the change in input signal light is inputted in an optical amplifier from the reverse direction with respect to the transmitting direction of the signal light, and output constant control of the optical amplifier is carried out by controlling pumping light of the optical amplifier response to the adjusting light. By this method, the range of input light power to be controlled in the optical amplifier can be made narrow, so that the response speed of the AGC can be enhanced easily (for example, refer to paragraphs [0016] to [0027] of the Patent Document 2).
However, at the technology described in the Patent Document 1, the optical fiber for delaying the signal light inputting in the optical amplifier delay is additionally disposed, thus resulting in the deterioration of characteristics and the large size of the optical amplifier due to this additional optical fiber. And at the technology described in the Patent Document 2, high speed control of the adjusting light is required, and this results in the increase of the power consumption and the heating of the optical amplifier by the adjusting light output.
It is therefore an object of the present invention to provide a controlling apparatus for an optical amplifier and a controlling method therefor, which can carry out a follow-up control with high speed response characteristic in response to the change in input power of signal light without the occurrence of the oscillation phenomenon, the larger size of the optical amplifier, and the increase of the power consumption and the heating.